Delaware Geological Survey Issues Report on Wastewater Treatment used for Rapid Infiltration Basin Systems
RI79 Simulation of Groundwater Flow and Contaminant Transport in Eastern Sussex County, Delaware With Emphasis on Impacts of Spray Irrigation of Treated Wastewater
This report presents a conceptual model of groundwater flow and the effects of nitrate (NO3-) loading and transport on shallow groundwater quality in a portion of the Indian River watershed, eastern Sussex County, Delaware. Three-dimensional, numerical simulations of groundwater flow, particle tracking, and contaminant transport were constructed and tested against data collected in previous hydrogeological and water-quality studies.
The simulations show a bimodal distribution of groundwater residence time in the study area, with the largest grouping at less than 10 years, the second largest grouping at more than 100 years, and a median of approximately 29 years.
Historically, the principal source of nitrate to the shallow groundwater in the study area has been from the chemical- and manure-based fertilizers used in agriculture. A total mass of NO3- -nitrogen (N) of about 169 kg/day is currently simulated to discharge to surface water. As the result of improved N-management practices, after 45 years a 20 percent decrease in the mass of NO3- -N reaching the water table would result in an approximately 4 percent decrease in the mass of simulated N discharge to streams. The disproportionally smaller decrease in N discharge reflects the large mass of N in the aquifer coupled with long groundwater residence times.
Currently, there are two large wastewater spray irrigation facilities located in the study domain: the Mountaire Wastewater Treatment Facility and Inland Bays Wastewater Facility. The effects of wastewater application through spray irrigation were simulated with a two-step process. First, under different operations and soil conditions, evaporation and water flux, NO3- -N uptake by plants, and NO3- -N leaching were simulated using an unsaturated flow model, Hydrus-1D. Next, the range of simulated NO3- -N loads were input into the flow and transport model to study the impacts on groundwater elevation and NO3- -N conditions.
Over the long term, the spray irrigation of wastewater may increase water-table elevations up to 2.5m and impact large volumes of groundwater with NO3-. Reducing the concentration of NO3- in effluent and increasing the irrigation rate may reduce the volumes of water impacted by high concentrations of NO3-, but may facilitate the lateral and vertical migration of NO3-. Simulations indicate that NO3- will eventually impact deeper aquifers. An optimal practice of wastewater irrigation can be achieved by adjusting irrigation rate and effluent concentration. Further work is needed to determine these optimum application rates and concentrations.
Ground-water levels are basic information needed for evaluating water conditions and for basic and applied research. For these efforts, water levels are being measured statewide in wells completed in multiple aquifers. Some wells are measured for specific projects, such as the Coastal Aquifers Salinity Project and the Water Conditions program, while other wells are measured so that staff can maintain long term records of ground-water levels for evaluation of trends. Table contains summary data from wells having 100 or more water level observations.
Groundwater levels are basic information needed for evaluating water conditions and for basic and applied research. For these efforts, water levels from various aquifers are being measured statewide. Some wells are measured for specific reasons, such as for the Coastal Aquifers Salinity Project and the Water Conditions Report, while other wells are measured so that staff can maintain long-term records of groundwater levels for evaluation of trends.
The geology and hydrology of the area between Wrangle Hill and Delaware City, Delaware, have been the focus of numerous studies since the 1950s because of the importance of the local groundwater supply and the potential environmental impact of industrial activity. In this report, 490 boreholes from six decades of drilling provide dense coverage, allowing detailed characterization of the subsurface geologic framework that controls groundwater occurrence and flow.
The region contains a lower section of tabular Cretaceous strata (Potomac, Merchantville, Englishtown, Marshalltown,and Mount Laurel Formations in ascending order) and a more stratigraphically complex upper section of Pleistocene-to-modern units (Columbia, Lynch Heights, and Scotts Corners Formations, latest Pleistocene and Holocene surficial sediments and estuarine deposits). The lowermost Potomac Formation is a mosaic of alluvial facies and includes fluvial channel sands that function as confined aquifer beds; however, the distribution of aquifer-quality sand within the formation is extremely heterogeneous. The Merchantville Formation serves as the most significant confining layer. The Columbia Formation is predominantly sand and functions as an unconfined aquifer over much of the study area.
To delineate the distribution and character of the subsurface formations, densely spaced structural-stratigraphic cross sections were constructed and structural contour maps were created for the top of the Potomac Formation and base of the Columbia Formation. The Cretaceous formations form a series of relatively parallel strata that dip gently (0.4 degrees) to the southeast. These formations are progressively truncated to the north by more flatly dipping Quaternary sediments, except in a narrow north-south oriented belt on the east side of the study area where the deeply incised Reybold paleochannel eroded into the Potomac Formation.
The Reybold paleochannel is one of the most significant geological features in the study area. It is a relatively narrow sandfilled trough defined by deep incision at the base of the Columbia Formation. It reaches depths of more than 110 ft below sea level with a width as narrow as 1,500 ft. It is interpreted to be the result of scour by the sudden release of powerful floodwaters from the north associated with one or more Pleistocene deglaciations. Where the Reybold paleochannel cuts through the Merchantville confining layer, a potential pathway exists for hydrological communication between Columbia and Potomac aquifer sands.
East of the paleochannel, multiple cut-and-fill units within the Pleistocene to Holocene section create a complex geologic framework. The Lynch Heights and Scotts Corners Formations were deposited along the paleo-Delaware River in the late Pleistocene and are commonly eroded into the older Pleistocene Columbia Formation. They are associated with scarps and terraces that represent several generations of sea-level-driven Pleistocene cut-and-fill. They, in turn, have been locally eroded and covered by Holocene marsh and swamp deposits. The Lynch Heights and Scotts Corners Formations include sands that are unconfined aquifers but complicated geometries and short-distance facies changes make their configuration more complex than that of the Columbia Formation.
To understand the effects of projected increased demands on groundwater for water supply, a finite-difference, steady-state, groundwater flow model was used to simulate groundwater flow in the Coastal Plain sediments of southern New Castle County, Delaware. The model simulated flow in the Columbia (water table), Rancocas, Mt. Laurel, combined Magothy/Potomac A, Potomac B, and Potomac C aquifers, and intervening confining beds. Although the model domain extended north of the Chesapeake and Delaware Canal, south into northern Kent County, east into New Jersey, and west into Maryland, the model focused on the area between the Chesapeake and Delaware Canal, the Delaware River, and the Maryland-Delaware border. Boundary conditions for these areas were derived from modeling studies completed by others over the past 10 years.
Compilation and review of data used for model input revealed gaps in hydraulic properties, pumping, aquifer and confining bed geometry, and water-level data. The model is a useful tool for understanding hydrologic processes within the study area such as horizontal and vertical flow directions and response of aquifers to pumping, but significant data gaps preclude its use for detailed analysis for water resources management including estimating flow rates between Delaware and adjacent states. The calibrated model successfully simulated groundwater flow directions in the Rancocas and Mt. Laurel aquifers as expected from the conceptual model. Flow patterns in the Rancocas and Mt. Laurel aquifers are towards local streams, similar to flow directions in the Columbia (water table) aquifer in locations where these aquifers are in close hydraulic connection.
Water-budget calculations and simulated heads indicate that deep confined aquifers (Magothy and Potomac aquifers) receive groundwater recharge from shallow aquifers (Columbia, Rancocas, and Mt. Laurel aquifers) in most of the study domain. Within shallow aquifers, groundwater moves toward major streams, while in the deep aquifers, groundwater moves
toward major pumping centers.
Quantifying Geologic and Temporal Controls on Water and Chemical Exchange between Groundwater and Surface Water in Coastal Estuarine Systems
Water Conditions Summary Groundwater Well Hydrographs
Delaware Water Conditions Report for current and historical periods of record.
This digital product contains gridded estimates of water-table (wt) elevation and depth to water (dtw) under dry, normal, and wet conditions for New Castle County, Delaware excluding the Piedmont. Files containing the point data used to create the grids are also included. This work is the final component of a larger effort to provide estimates of water-table elevations and depths to water for the Coastal Plain portion of Delaware. Mapping was supported by the Delaware Department of Natural Resources and Environmental Control and the Delaware Geological Survey.
These grids were produced with the same multiple linear regression (MLR) method as Andres and Martin (2005). Briefly, this method consists of: identifying dry, normal, and wet periods from long-term observation well data (Db24-01, Hb14-01); estimating a minimum water table (Sepulveda, 2002) by fitting a localized polynomial surface to elevations of surface water features (e.g., streams, swamps, and marshes); and, computing a second variable in the regression from water levels observed in wells. Separate MLR equations were determined for dry, normal, and wet periods and these equations were used in ArcMap v.9 (ESRI, 2004) to estimate grids of water-table elevations and depths to water. New Castle County was divided into a northern section and a southern section with the C&D Canal being the natural line of demarcation. A minimum water-table surface was then calculated for both the northern and southern sections of New Castle County. However, dividing the county, as well as the water-level data, into two sections did not result in sufficient regression coefficients for use in the estimation process. Therefore, the data (minimum water-table surface and water-level data) were merged together and the water-table elevation and depth to water grids for dry, normal, and wet conditions were then calculated for the county as a whole.
This digital product contains gridded estimates of water-table (wt) elevation and depth to water (dtw) under dry, normal, and wet conditions for Kent County, Delaware. Files containing the point data used to create the grids are also included. This work is the final component of a larger effort to provide estimates of water-table elevations and depths to water for the Coastal Plain portion of Delaware. Mapping was supported by the Delaware Department of Natural Resources and Environmental Control and the Delaware Geological Survey.
These grids were produced with the same multiple linear regression (MLR) method as Andres and Martin (2005). Briefly, this method consists of: identifying dry, normal, and wet periods from long-term observation well data (Hb14-01, Jd42-03, Mc51-01, Md22-01); estimating a minimum water table (Sepulveda, 2002) by fitting a localized polynomial surface to elevations of surface water features (e.g., streams, swamps, and marshes); and, computing a second variable in the regression from water levels observed in wells. A separate MLR equation was determined for dry, normal, and wet periods and these equations were used in ArcMap v.9 (ESRI, 2004) to estimate grids of water-table elevations and depths to water. Kent County was divided into three regions (south, central, north). A minimum water-table surface was calculated for each of these areas and were merged together to create a single minimum water-table surface for the entire county. This grid was filtered and smoothed to eliminate edge effects that occurred at the boundaries between each of the three regions. Water-table elevation and depth to water grids for dry, normal, and wet conditions were then calculated for the county as a whole.